In a server or a personal computer, a heat spreader is fastened to an electronic component such as a CPU (Central Processing Unit) to radiate heat generated by the electronic component to the outside.
If the thermal resistance between the heat spreader and the electronic component is high, it is not possible to rapidly transfer the heat of the electronic component to the heat spreader. For this reason, a heat radiation sheet having excellent thermal conductivity is sometimes provided between the electronic component and the heat spreader.
Various types of heat radiation sheets are available. An indium sheet is one example of the heat radiation sheets. Since an indium sheet uses expensive indium, it is difficult to reduce the cost of the heat radiation sheet.
In this respect, as a heat radiation sheet alternative to the indium sheet, a technology using carbon nanotubes has been investigated. In this technology, a plurality of carbon nanotubes are provided upright on a sheet to transport heat from one end to the other end of each carbon nanotube.
The carbon nanotubes have thermal conductivities of about 1500 W/m·K to 3000 W/m·K, which are higher than the thermal conductivity of indium (80 W/m·K). Hence, it is preferable to use carbon nanotubes for a heat radiation sheet.
The temperature of an electronic component in contact with a heat radiation sheet varies depending on the condition of use of the electronic component, and the electronic component deforms with the change in temperature. If the heat radiation sheet is excessively thin, the deformation of the electronic component causes the heat radiation sheet to be separated from the electronic component, making it difficult to transport heat from the electronic component to the heat radiation sheet.
To prevent this, it is effective that the thickness of the heat radiation sheet is increased to some degree by increasing the length of each carbon nanotube, and moreover the heat radiation sheet is brought into close contact with the electronic component by applying pressure at assembly.
However, when the length of the carbon nanotube is increased as described above, the carbon nanotube as a whole becomes soft. For this reason, there arises a possibility that the carbon nanotubes do not withstand the pressure at assembly and may collapse.
In addition, if the carbon nanotubes become soft as described above, there also arises a possibility that the carbon nanotubes do not follow the deformation of the electronic component with the change in temperature.
To avoid these problems, it is conceivable that the carbon nanotubes are coated with mechanically strong films to reinforce the carbon nanotubes. However, it is difficult to coat the entirety of a long carbon nanotube by an already-existing technology.
Note that technologies related to the present application are disclosed in Japanese Laid-open Patent Publication Nos. 06-5754, 2003-174127, 2013-211430, 2005-150362, 2006-147801, and 2006-303240.